LONDON — Russian Alexander Litvinenko was a “registered and paid” agent working for Britain’s foreign intelligence agency when he died after being mysteriously poisoned, a lawyer representing his widow told an official hearing Thursday. Another lawyer said the U.K. has evidence that the Russian government was behind Litvinenko’s death.

Britain is investigating the demise of Litvinenko, who died in November 2006 after drinking tea laced with the rare radioactive isotope polonium-210 at a London hotel. The case has badly strained relations between the United Kingdom and Russia, which denies poisoning the former Russian agent-turned-Kremlin critic.

Thursday’s session aimed to set out the scope of a public inquest into Litvinenko’s death. Judge Robert Owen said the inquest is expected to start in May.

Lawyer Ben Emmerson said that, at the time of his death, Litvinenko was working for Britain’s MI6 spy agency and had been tasked to help Spanish intelligence investigate the Russian mafia. The U.K. probe must consider if MI6 failed to properly assess the risks before sending the agent out on his assignment, Emmerson said.

Lawyer Neil Garnham, representing Britain’s Home Office, told the hearing he could “neither confirm nor deny” if Litvinenko was employed by British intelligence.

Meanwhile, Hugh Davies, the lawyer who advises the coroner in the inquest, told the hearing that a “high-level assessment” of confidential material provided by the British government established a case for the Russian state’s culpability in Litvinenko’s poisoning.

Related

Litvinenko, a former Russian FSB agent, blamed the Kremlin — specifically Russian President Vladimir Putin — for his impending death, and his family has demanded Russian authorities be held accountable. Britain has accused two Russians, Alexander Lugovoi and Dmitry Kovtun, of killing Litvinenko, but Moscow has refused to hand them over.

Inquests are held in Britain to determine the facts whenever someone dies unexpectedly, violently or in disputed circumstances. Inquests are meant only to determine a cause of death, so they don’t apportion blame. But in Litvinenko’s case every detail of the inquiry is being scrutinized for clues to the alleged involvement of Russia’s secret services.